A conventional hologram recorder is disclosed in Patent Document 1 for example. The hologram recorder disclosed therein records holograms in a hologram recording medium by means of a phase code multiplexing method. Laser light which comes out of abeam source is split by a beam splitter into signal light (signal beam) and reference light (reference beam). The signal beam is modulated by a spatial light modulator into a beam of a pattern according to the information to be recorded, and then directed to the hologram recording medium. The reference beam is modulated by a phase encoding multiplexer into a beam of an appropriate phase pattern thereby given a phase code. The phase encoding multiplexer, which is a transparent liquid-crystal device having a large number of cells, gives the incoming reference beam a predetermined phase difference per each cell, thereby modulating the reference beam into a transmitted beam which has a desired phase pattern. After the phase modulation, the reference beam is directed to the hologram recording medium so that the beam will overlap with the signal beam in the hologram recording medium. As a result, a hologram which is made of interference stripes (page pattern) of the signal beam and the reference beam is recorded. In this process, if the modulation patterns (pixel pattern or phase pattern) of the signal beam and of the reference beam are varied without changing the beam irradiation site on the hologram recording medium, holograms of various page patterns according to the modulation patterns are recorded in a multiplex manner in the beam irradiation site. This is how the phase code multiplexing method is implemented.                Patent Document 1: JP-A-H07-20765        
However, the above-described conventional hologram recorder is designed to have an optoelectrical characteristic: Namely, in each of the cells in the liquid-crystal device through which the reference beam passes, a phase difference is generated in accordance with a drive voltage, through the use of birefringence. In such an arrangement where birefringence is utilized in order to generate a discretional phase pattern, a complicated voltage application control must be performed for each cell, resulting in complication in terms of configuration and workings for the phase modulation.
In the case of e.g. a typical transmissive liquid-crystal device using nematic liquid crystal molecules, very fine variable control, like a gradation expression control, must be performed for voltage application per each of the cells every time the phase pattern is varied. In addition, the response speed of liquid crystal molecules is not very high, which leads to slow hologram recording per page. In this respect, there has been room for improvements.